Technical Field
This invention relates generally to electrostatographic process machines such as a copier or printer for producing toner images on receiver sheets, and more particularly to a fusing apparatus, in such a machine, which includes a receiver sheet decurling mechanism and a method of decurling a toned sheet.
Electrostatographic process machines such as copiers and printers, which, for example, produce or reproduce toned images on selected substrates by employing electrostatic charges and toner particles on an image-bearing surface (IBS) such as a photoconductive surface are well known. Typically, such machines operate through a sequence of currently well known electrostatographic process steps. In a copier or printer type of such machines, for example, these steps include (1) charging of an insulated photoconductive surface with electrostatic charges, (2) forming a latent image electrostatically on such surface by selectively discharging areas on such surface, (3) developing the electrostatic image so formed with particles of toner, (4) transferring the toned image to a suitable receiver sheet for fusing thereon by a fusing apparatus to form a hard copy, and (5) cleaning the photoconductive surface by removing residual toner and/or other particles therefrom in preparation for similarly reusing such surface to produce another such image.
In the above process, the quantity of toner particles, which forms each toned image on an imaging frame of the IBS, depends significantly on the quantity or level of charge on the image bearing member just before image development. The quality of the fused hard copy obtained depends in part on the condition, for example the flatness, of the receiver sheet especially following the fusing step.
Unfortunately however, in an electrostatographic machine that includes a fusing apparatus having a soft surface heated fuser roller that forms a fusing nip with a hard surface pressure roller, fused receiver sheets tend to curl undesirably following the fusing step. For example, a receiver sheet which has a substantially heavy quantity of toner particles forming the image transferred thereto when fused by such apparatus usually tends to have an undesirable concave curl in the toner image-carrying side of such sheet. On the other hand, a receiver sheet which is carrying a lightly toned image usually has instead an undesirable convex curl in the image-carrying side thereof when fused by such apparatus. Such concave and convex curls are undesirable not only because they detrimentally affect the quality and appearance of the finished hard copy, but also because such curls make handling of the receiver sheets difficult thereafter.